1. Field of the Invention
The present invention generally relates to map data distribution systems, and more particularly to a system for a transportable device to acquire a necessary detailed map from a base station. The present invention also relates to a map data acquisition device mounted to the transportable device and suited for use in such system. The expression "detailed map" means a map with detailed information about a relatively small area. Maps other than the detailed map, i.e. general maps, may be pre-stored in the vehicle. The present invention is suitable for use in a navigation device for vehicles.
2. Description of the Related Art
As electronic devices that utilize map data, navigation devices are well known. Such navigation devices commonly present a map of the area around the current position of a vehicle and perform route guidance to a destination using the map data. Generally, such a navigation device is provided with a storage medium (such as a CD-ROM) having map data stored therein, from which necessary map data is read out.
However, to have the medium on which is stored detailed maps for a wide area available, requires always carrying rarely used maps, which can be very inefficient. Therefore, a type of navigation device has been proposed that acquires the detailed map of a given area from an external source when such map is needed. This type of system is disclosed, for example, in Japanese Patent Laid-Open Publication No. Hei 7-262493.
According to the system disclosed in the above publication, a general nationwide map including main roads is stored in a transportable device and a user instructs the transportable device to acquire a detailed map of a desired area. In response to the instruction, the transportable device obtains the detailed map from a base station. When route guidance is to be carried out, following operations are performed. When a user at a starting point inputs a destination, a detailed map of the area surrounding the destination is acquired. When the vehicle begins travelling, route guidance is initiated using the general map and the detailed map for the area surrounding the destination.
However, even though the detailed map of the destination is acquired at the starting point, a great distance is left before reaching the destination. If the destination is modified on the way, the detailed map acquired through communication processes will be wasted. Even if the destination is not modified, the detailed map of the destination occupies a portion of the memory capacity during the travel, which is not preferable in terms of efficient memory use. With this system, the information which is not used for a while must be kept in the memory, resulting in the need for increase in memory capacity.
The traveling conditions continuously change while the vehicle is running. For example, a vehicle may deviate from the route established to reach the destination. In a conventional system, a user must enter an instruction him/herself to acquire a large scale map of the area where the vehicle has entered. However, the user may not know where it has started to deviate or for which area the detailed map is needed in order to return to the established route. Thus, it is not easy for a user to decide which detailed map is necessary to cope with the change in traveling conditions. Even if a user can make such decision, giving an instruction to acquire the necessary detailed map is troublesome for the user.
In order to avoid the above problems, one can envision acquisition of the detailed maps for all the areas the vehicle travels through before reaching the destination. However, this increases the load of the communication device and interferes with communication of other types of information (such as traffic information), and also leads to an increase in required memory capacity of the transportable device. Since a great number of unnecessary maps are also acquired, an advantage of obtaining only the necessary data cannot be enjoyed.
As described above, according to conventional systems, detailed maps are automatically acquired based on the positional information regardless of the traveling conditions. As a result, it is often the case that unnecessary detailed maps are acquired and that such acquisition is made too early. The map which is truly necessary to cope with the change in traveling conditions cannot be acquired without a user's instruction. Consequently, the data storage space of the transportable device cannot be efficiently utilized, and also useful detailed maps cannot be acquired at the appropriate time. The above-described problems are not limited to vehicle-mounted devices but are encountered in other types of transportable devices as well.